Alteration of circadian rhythm in shift-working ambulance personnel. Monitoring of salivary cortisol rhythm.

In order to investigate the relationship between internal desynchronization and clinical intolerance to shift work, a set of circadian rhythms including salivary cortisol rhythm were monitored in seven shift-working ambulance personnel. Oral temperature, grip strength of both hands, subjective assessment of drowsiness, fatigue and attention were recorded approximately every four hours except during sleep, for seven days. Self-sampling of salivary cortisol, which was suitable evaluation of endocrine rhythm in field studies, was also tried in this study. cosinor (cosine curve fitting) method and power spectral analysis were used for time series data analysis. The internal desynchronizations between rhythms of physiological variables and sleep-wake cycle were observed in shift-working ambulance personnel. The incidence of internal desynchronization seemed to be higher in intolerant subjects, although the difference between tolerant and intolerant subjects did not reach a statistically significant level. The peak time of salivary cortisol rhythm during a twenty-four hour shift was phase-advanced as intolerant to shift work showed apparently an atypical circadian pattern of salivary cortisol with an abnormal peak at 21.00 h. In this subject, the clinical intolerance to shift work seemed to be associated with internal desynchronization of circadian rhythms. The present study confirmed the internal desynchronization of the circadian rhythm in physiological rhythms like oral temperature and grip strengths, and was in favour of the hypothesis of an internal desynchronization and clinical intolerance to shift work. The clinical implication of impairment of salivary cortisol rhythm remains to be further investigated.